Confined parallel rail pulsed plasma accelerator



Jan. 17, 1967 M. E. MAES 3,298,179

CONFINED PARALLEL RAIL PULSED PLASMA ACCELERATOR Filed NOV. 12. 1963 2 Sheets-Sheet l Pan 2 J'I/PPLY 201 x x x x r 2 F x x x x A 4-- Z5 Q2 24 l x x 2a 1 X X K (Ame/roe INVENTOR. MICHEL 5. H455 ATTORNEYS Jam. 17, 1967 Filed Nov. 12, 1963 M. E. MAES CONFINED PARALLEL RAIL PULSED PLASMA ACCELERATOR 2 Sheets-Sheet 2 INVENTOR.

[United States Patent 3,298,179 CUNFINED PARALLEL RAIL PULSED PLASMA ACCELERATOR Michel E. Macs, Eellevue, Wash, assignor to Rocket Research Corp, fieattle, Wash., a corporation of Washington Filed Nov. 12, 1963, Ser. No. 322,862 21 Claims. (Cl. 60-202) This application is a continuation-in-part of my copending and now abandoned application Serial No. 228,- 854, filed October 8, 1962, hearing the same title.

The invention in general relates to reaction propulsion engines and methods of operating same, and more particularly to so-called pulsed plasma accelerators having particular utility for satellite attitude control and station keeping and for primary propulsion of deep space probes and interplanetary space vehicles, for example.

Reaction propulsion engines according to the present invention involve a pulsed plasma accelerator comprising spaced electroconduct-ive rails within an insulative container providing a plasma channelling chamber closed at one end and open at the other, the said chamber being laterally of a width considerably greater than the thickness thereof, with the sides of the container being substantially flat and closely spaced with respect to the rails, said accelerator further comprising means including an energy storage capacitor for charging and recharging one of said rails to a high voltage with respect to the other and means for injecting a vaporizable and ionizable propellant into the closed end of said container.

In general, also, the preferred mode of operation of such engines is what may be termed autogenous, or continuously repetitively self-pulsing, Le. a mode of operation comprising continuously applying to an energy storage capacitor connected across said rails a relative potential difference of sutficient magnitude to cause propellant ionization between said rails, with continuous injection of the propellant into the closed end of said chamber.

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In certain preferred variations such engines involve a ferromagnetic sheath laterally surrounding the insulative container in which the rails are housed, the ferromagnetic sheath being configured in a manner placing magnetic pole pieces substantially against the side faces of the container, so that a high flux density is self-induced within the container, the self-induced magnetic lines of force extending parallel to the thickness dimension of the container and thus being oriented so as to accelerate the plasma.

Pulsed plasma accelerators according to the present invention are ideally suited for grouping or clustering to provide an attitude control system, with a plurality of plasma accelerating channels respectively arranged along differently directed thrust axes to provide selective control of space craft attitude. Such a system, involving grouping or clustering of accelerators, can advantageously include a single energy source associated with various or all rail pairs and a single energy storage capacitor associated with various or with all of the rail pairs.

Known reaction engines of the pulsed plasma accelerator type, or so-called plasmoid guns, have involved inherently low operating efficiency as a result of inefiicient utilization of propellant and/or inefficient generation of "ice magnetic field. Basically these guns have been of two types, the so-called rail gun and the so-called concentric cylinder gun. Reference may be had, for example, to the Corliss text entitled Propulsion Systems for Space Flight, published in 1960 by McGraw-Hill Book Co., New York, New York, at ages 219 and 22 0, for a more specific disclosure of these prior types of plasma accelerators.

For any given current magnitude and any given electrode spacing, a parallel rail configuration has been shown to produce maximum magnetic field acting on the plasma. However, the simple parallel rail configuration, without an insulative container surrounding the electrodes,'allows large quantities of propellant to escape prior to and during .plasma acceleration. Other pulsed plasma accelerator configurations, notably the so-called coaxial or otherwise the concentric cylinder gun configuration, also known as the Patrick gun, encloses the propellant but sacrifices considerable magnetic field, reducing the momentum obtained for any given discharge, and consequently lowering the reaction efficiency. Furthermore, the required large cross-sectional areas of the concentric cylinder type accelerator result in a relatively rather large accelerator size and weight. The concentric cylinder type of plasma accelerator is theoretically and experimentally a very different type of accelerator than a rail gun with confined rails as contemplated by the present invention. In the concentric cylinder type gun the ionized plasma, once formed, travels between non-conductive cylinders and not between electrodes. This gives rise to a basic diiference in the mechanism of plasma acceleration and the mode of operation of the respective guns, since in the rail type gun the electrodes remain in circuit throughout the acceleration of the plasma.

All known previous rail type guns have always been operated without any confining cover or channeling means for the plasma, with resulting poor etficiency. Addition of a channeling insulative cover or container of small thickness dimension and extending substantially the entire length of rail-type electrodes, in the manner characteristic of the present invention, markedly changes the performance characteristics of the gun, rendering it much more efficient than either the concentric cylinder type gun or the unconfined rail type gun. The unconfined rail type gun has been known and the subject of widespread experimentation since about 1956, and has always been considered to have an inherently low performance characteristic because of various mechanical limitations, principally the escape of propellant upon admission of the propellant to the electrode region. The addition of cover means to an unconfined rail type gun solves the problem of loss of plasma and can increase the efiiciency of the rail type gun to close to its theoretical capability. By way of example, the order of efiiciency of an unconfined rail gun is about 310% of theoretical, while a confined rail gun such as shown at FIGS. 2-4 hereof has an operating efiiciency of about 50-60% of theoretical and even considerably higher when used in conjunction with means providing a self induced augmented magnetic field providing increased plasma acceleration, such as shown at FIGS. 7, 7A and 7B thereof.

As to use of a plasma confining cover around the electrode rails of a rail type gun, it is to be emphasized that merely providing a cover (such as the cylindrical cover of the concentric cylinder type gun) does not of itself provide increased efficiencies of this order. The plasma, for good propulsive efficiency, must be confined to the zone directly between the electrodes. The covered rail type gun presented by the present invention defines a cross-sectional area always filled by the plasma arc,

a, a and the current must always flow through the arc. It is unique to confined rail guns of the present invention that an operating environment is provided which requires current flow through substantially all of the plasma and, by virtue of this functional relationship, all of the propellant is made to take part in the mass flow generating discharge. Neither a concentric cylinder type gun nor an unconfined rail type gun has this capability. In the case of the concentric cylinder type gun, the discharge is quite prone to spiking, i.e. radially concentrated discharges which lead to instability because the locale of the first radial discharge becomes the current path of least impedance, with little or no ionization of the gas along other radial paths. In contrast, the confined rail gun with its narrow chamber advantageously utilizes the spiking phenomenon to maintain a high density, small cross-sectional area discharge. As a direct outgrowth of this, smaller and lighter guns are possible than heretofore, without any loss of efliciency because of size. It is also to be noted that, with respect to the distinctions between the unconfined rail type gun and the concentric cylinder type gun it is geometrically impossible, from a practical point of view, to produce an acceleration augmenting magnetic field in a concentric cylinder type gun. More specifically, for example, the magnetic field generated by the bias coil of the Patrick concentric coil cylinder type gun is in the wrong direction to assist plasma acceleration. The magnetic field in the Patrick gun causes the electrons in the plasma to drift in a compound circular direction to assist in maintaining a more uniform discharge. In the Patrick gun,. such magnetic field also tends to keep ions from the outer cylindrical surface and thus reduce drag. But neither of these functions can be categorized as true magnetic acceleration of the type where the lines of flux of the magnetic field lie across the direction of plasma flow.

Discussions of some of the important parameters for a practical pulsed plasma accelerator and a discussion of the various types of configurations, as well as ancillary considerations pertaining to the present invention, are also presented in my technical articles on the subject, as follows:

1) The Pulsed Plasma Accelerator for Space Propulsion, presented to the National Aeronautic and Space Engineering and Manufacturing Meeting, Los Angeles, California, on October 9-13, 1961, available as Preprint No. 419C, published by the Society of Automotive Engineers, Inc., 485 Lexington Ave., New York 17, N.Y.;

(2) Rail Proposed for Attitude Control appearing in Missiles and Rockets, vol. 10, No. 6, issue of February 5, 1962, at pages 34-36;

(3) The Confined Parallel Rail Pulsed Plasma Accelerator, a paper presented to the American Rocket Society Electric Propulsion Conference, Berkeley, California, on March 14-16, 1962, available as American Rocket Society Preprint No. 2,397-62;

(4) Experimental Investigation of the Confined Parallel Rail Pulsed Plasma Accelerator, Proceedings of the Third Annual Symposium on the Engineering Aspects of Magnetohydrodynamics, Rochester, New York, March 28-29, 1962; and

(5) Pulsed Plasma Accelerator Efiiciency Improvement, appearing in the 1962 Proceedings of the National Aerospace Electronics Conference, Dayton, Ohio, May 14-16, 1962, at pages 635-641.

To the extent they pertain to the subject matter of this invention, the disclosures of these articles are incorporated herein by reference.

The various objects, features and advantages of the present invention include the following:

(1) Practical usage of spaced, parallel rails for maximal magnetic field production.

(2) Usage of enclosed or contained parallel rails so that propellant mass loss before and during the acceleration of the plasma is avoided without sacrifice of high [5,. magnetic field production, with resulting maximization of impulse delivery and efficiency.

(3) The provision of pulsed plasma accelerators of narrow cross-section and relatively small size and therefore low weight, without loss in efficiency because of small size.

(4) Provision in a pulsed plasma accelerator of a narrow plasma channeling container, the configuration being such that the plasma arc fills the entire channel crosssection and substantially all of the propellant is acted upon by the electrical discharge.

(5) Provision of a pulsed plasma accelerator of confined, narrow cross-sectional area, which allows a mode of rapid pulsing and continuous operation such that propellant is continuously admitted to the accelerating channel and the energy storage capacitor is continuously recharged after every discharge, such operation being characterized by a series of autogenously repetitive, i.e. self-pulsing, discharges.

(6) Provision of a further enhancement of such series of pulsed discharges by use of a propellant flow modulating means to improve uniformity of pulsing rate and overall reaction efficiency.

(7) Provision of a confined parallel rail accelerator of narrow cross-sectional area allowing the use of a ferromagnetic sheath about the insulative container confining the rails, the magnetic flux across the plasma path being self-induced and the flux density thereof being greatly increased and made more uniform by the ferromagnetic sheath, thus further improving impulse and reaction efficiency.

(8) The provision of a'lightweight, narrow accelerating channel which is easily capable of being clustered to form attitude control propulsion packages by which a space crafts attitude may be cont-rolled simply by selective pulsing of the proper accelerating channel.

(9) Usage of a single energy source, single energy storage capacitor, and single propellant supply in conjunction with a plurality of confined rail pairs to supply the necessary high voltage pulse of energy and propellant to any selected accelerating channel.

(10) The provision of pulsed plasma accelerators which are constructionally very simple and very easily fabricated.

(11) The provision of pulsed plasma accelerators of minimal size and weight and maximal simplicity which are nonetheless operationally highly reliable.

(12) The provision of pulsed plasma accelerators capable of using a large number of propellant materials, and capable of providing a deep space propulsion system with considerably less total weight than equivalent chemical systems.

(13) The provision of a pulsed plasma accelerator configuration having the characteristic of maintaining accurately repeatable impulse bits even with substantial variation in the amount of propellant mass admitted per pulse.

These and other objects, features, advantages and characteristics of the present invention will be apparent from the following specific description of various typical embodiments thereof taken together with the accompanying drawings wherein like numerals refer to like parts and wherein:

FIG. 1 is a diagrammatic presentation showing the operation of a parallel rail pulsed plasma accelerator;

FIG. 2 isa plan view, with various parts broken away, of a typical confined parallel rail pulsed plasma accelerator, taken substantially along line 2-2 of FIG. 4;

FIG. 3 is a view in side elevation of the confined parallel rail accelerator shown in FIG. 2;

FIG. 4 is an end view of the confined parallel rail accelerator shown in FIGS. 2 and 3;

FIG. 5 is a cross-sectional view of a typical propellant flow modulator;

FIG. 6 is a fragmentary view in longitudinal crosssection and on an enlarged scale of a modified form of confined parallel rail pulsed plasma accelerator, incorporating a flux concentrator in the form of a ferromagnetic sheath, said view being taken substantially along line 66 of FIG. 7;

FIG. 7 is a view in lateral cross-section of the accelerator shown in FIG. 6, taken substantially along line 7-7 thereof;

FIG. 7A is a view in lateral cross-section showing a fragment of a modified form of confined rail accelerator according to the invention, wherein the electrode covering includes a flux concentrator in the form of a sheath of packed iron powder sheathed in epoxy resin;

FIG. 7B is a fragmentary cabinet view of yet another form of confined rail accelerator and associated flux concentrating sheath, the ferromagnetic sheath in this modification being composed of a laminate of thin iron sheets and interleaved insulative sheets, with various portions of of the cluster shown in FIG. 8, taken substantially along line 9-9 thereof; and

FIG. 10 is an isometric view illustrating typical usage of. clusters of confined rail accelerators for attitude control of a space vehicle.

The operation of a pulsed plasma accelerator is diagrammatically illustrated in FIG. 1. Such accelerator in general functions to produce thrust from stored electrical energy. The basic accelerator configuration involves two spaced, elongate substantially parallel electrodes or rails 20, 22 which are connected across an electrical energy storage capacitor 24 and a high voltage power supply 26. When the capacitor 24 is charged, one of the rails is at a high voltage with respect to the other. No electrical breakdown occurs because these rails are located in a vacuum. However, when a small amount of propellant is injected or otherwise placed between the rails, a plasma column or blob 28 of ionized propellant forms between the electrodes 20, 22 and the electrical energy stored in the capacitor 24 discharges across the electrodes through the plasma column 28. The direction of current I in the electrodes 20, 22 during the discharge between the electrodes 22 (as designated by arrows 20, 22) creates a reinforced magnetic field 30 in the region between said electrodes 20, 22 (as designated in FIG. 1 by crosses). The direction of the magnetic field 30 is perpendicular to the direction of the current I flowing in the plasma column 28 (which latter current is designated by arrow 28'). The interaction between the perpendicular magnetic field 30 and plasma current I (arrow 28) creates a mutually perpendicular force accelerating the plasma column 28 down the electrodes 20, 22 and out of the accelerator, such accelerating force being shown by the arrow designated F. As will be understood, the accelerating force F results in an equal and opposite reaction force F acting upon the electrode portions parallel to the plasma column 28 at the closed end of the accelerator. The reaction force F and accompanying designating arrows are also shown in FIG. 1.

It is to be noted that since the plasma column 28 remains electrically neutral during the entire acceleration process, electrons do not have to be added to the ac celerator exhaust. It is thus possible to produce thrust directly from electrical energy using electromagnetic forces and a neutral plasma. Furthermore, the amount of kinetic energy that can be added to the plasma is practically unlimited and specific impulses of between about 2,000 and 10,000 seconds or more have been produced by this type of accelerator.

Important advantages can be attributed to a pulsed plasma accelerator as compared with so-called ion certain of the laminant sheets broken'away to more cleartype accelerators. Since the plasma accelerator is a pulsed device, it is possible to vary the thrust thereof by a factor of 1,000 or more by simply varying the pulsing rate. It is also possible to vary the specific impulse by varying either the amount of the admitted propellant or the quantum of energy discharged per pulse. Furthermore, the accelerator can be turned On or off instantly with no loss of propellant or power. In addition, a pulsed plasma accelerator can utilize a wide variety of easily stored and inexpensive liquified gases or solids as the propellant. And, of considerable operational importance, the pulsed plasma accelerator is of simple and rugged construction and therefore capable of high reliability for long duration missions, while the ion type accelerator inherently requires rather close electrode spacings and delicate construction. Typical construction of a confined parallel rail pulsed plasma accelerator is illustrated in FIGS. 2-4. The elongate substantially parallel electrodes 32, 34 are enclosed in an insulative container 36 extending substantially the length of the electrodes and the electrodes are connected to an energy storage capacitor 38 by a short conical coaxial lead 40. Such a coaxial lead for example serves both as a low inductance electrical connection between the capacitor 38 and the electrodes 32, 34 and also can serve to distribute heat from the electrodes 32, 34 to the outside case of the capacitor 38.

The container 36, as shown, is closed at one end and open at the other, and can be fabricated from any suitable insulative material. In the form shown, insulative container 36 in which the electrodes 32, 34 are housed is molded exteriorly around the electrodes and constituted of a suitable plastic or ceramic composition. As will be apparent, however, the container 36 in order to function in a desired manner can take various other forms, such as simply a pair of glass or ceramic plates placed against the electrodes.

Propellant is admitted in a small hole 42 in the rear of the enclosing container 36. The container in effect provides a plasma channeling chamber having a rela tively narrow, confined volume extending substantially the entire length of said electrodes 32, 34. FIGS. 3 and 4 typically illustrate the narrowness of said channel.

Mounted above said insulating cover 36 and in direct communication with hole 42 is a modulator means 44 (also see FIG. 5).

FIG. 4 illustrates a lateral cross-section of the accelerator and typically illustrates the close spacing of the insulative container 36 about the two electrodes 32, 34. As shown in FIG. 4, the plasma chamber has a crosssection lateral to the direction of plasma movement which is bounded by electroconductive surfaces and insulative surfaces in alternating pattern, with the total surface area of said insulative surfaces being substantially greater than the total surface area of said electroconductive surfaces.

The confined parallel rail pulsed plasma accelerator may, for example, operate in either of two modes. The first mode consists of pulses on command in which single discrete pulses of propellant are admitted to the accelerato-r which will initiate a single discharge. The second mode consists of a continuous sustained series of repetitive pulses occurring as propellant is admitted in a continuous stream, with the energy storage capacitor allowed to recharge continuously. This continuous repetitive pulsation is best described as a cyclic sweeping clear of propellant from the plasma confining chamber by the discharge and refilling of said chamber by continuous propellant admission. The energy storage capacitor is recharged by the high voltage power supply that is continuously attached to it, and such recharging continues until sufficient propellant has refilled said chamber to cause another discharge. Such automatically continuous, repetitive pulse mode of operation can suitably occur at a rate on the order of 1,000 pulses per second.

a formation.

Propellant admission to the pulsed plasma accelerator can be accomplished by any of the following techniques: (1) admission of a gas or vapor on command by means of a rapidly opening and closing valve, resulting in single pulse operation; (2) admission of a continuous stream or flow of gas or vapor, resulting in sustained repetitive pulsing operation; (3) admission of a continuous stream of gas or vapor which is cyclically modulated by a flow modulator such as an oscillating flapper valve; (4) insertion of a Wire or other solid propellant by repetitive mechanical means; (5) storage of propellant in solid form in the accelerating chamber, which erodes in suflici-ent quantity to make up the plasma column; (6) storage of propellant in solid form in the accelerator chamber, including the use of the electrodes and/ or insulating container as propellant material eroding in sufiicient quantity to make up the plasma column; and (7) a combustible combination of liquids or gases, wherein the combustion reaction increases the propellant temperature and enhances ionization thereof, i.e. promotes plasma In certain adaptations of pulsed plasma accelerators, propellant ionization can also be enhanced by placing in the accelerating chamber a radioactive material. Typical ionizable and vaporizable propellants are: water vapor, ammonia, carbon dioxide, nitrogen, oxygen, hydrogen, air, argon, helium, copper, Nichrome and steel.

FIG. 5 is illustrative of a typical cyclic flow modulator for gaseous propellant. A vibrating spring or flapper member 46 is mounted within a narrow, non-magnetic valve body 48 and is vibrated by a suitable solenoid 50, which is in turn actuated in a manner determined by the desired mode of operation. The action of magnetic forces created by the solenoid 50 on the oscillating flapper 46 causes it to vibrate up and down which alternately opens and closes the discharge orifice 52. Propellant enters the modulator 44 through the tube 54 and passes out of the modulator 44 in cyclic pulses. The flow modulator 44 preferably serves to create pulsations or chops in a continuous propellant flow rather than deliver discrete pulses of propellant.

Cyclically pulsing the propellant improves the control of the pulsing rate and control of the voltage to which the energy storage capacitor charges, with the result that propellant loss between capacitor discharges is significantly reduced.

A flow modulator or flow interrupter having a piezoelectric ceramic element to cyclically constrict the propellant flow path can also be employed. With use of a piezoelectric modulator element, a tap into the capacitor charging voltage can be used to actuate the piezoelectric element, so that capacitor charging and propellant bursts are synchronized. A magnetostrictive propellant flow modulating element can likewise be employed, suitably with magnetostrictive element actuation occurring responsively to the magnetic field produced by the previous current discharge.

FIGS. 6, 7, 7A and 7B illustrate various typical applications of a magnetic fiux concentrating ferromagnetic sheath about a confined parallel rail accelerator. In these forms of the invention, the elongate electrodes 32, 34 and the insulative container 36 are surrounded by a ferromagnetic sheath or core. Specifically, in the form shown at FIGS. 6 and 7, core 60 is an epoxy resin impregnated with particulate magnetic material such as dispersed iron particles, the iron content being about 90% by weight, for example. The ferromagnetic sheath 60 serves as a flux concentrator and greatly increases and makes more uniform the self-induced magnetic field produced by the current in electrodes 32, 34 and in the plasma column, by virtue of the fact that the concentrator pole pieces (sheath faces 62, 64) are closely spaced to each other across the current flow path. Thus the plasma zone between electrodes 32, 34 surrounded by the insulative cover in which the plasma column is confined E5 and accelerated is subject to a much higher magnetic flux or field than would otherwise occur. FIG. 7 is a lateral cross-section of the embodiment shown at FIG. 6, further illustrating the arrangement of electrodes 32, 34, insulative container 36, and iron powder-resin ferromagnetic magnetic fiux concentrating sheath 60. FIG. 7 also further illustrates the relative close spacing of the ferromagnetic sheath pole pieces '62, 64.

FIG. 7A illustrates a modified form of flux concentrating ferromagnetic sheath used in conjunction with the electrode rails 32, 34 and the insulative container 36. Specifically, in this form of the invention, the ferromagnetic sheath 60 is in the form of compressively packed or sintered iron particles within an epoxy resin protective coating 61.

FIG. jB illustrates another modified form of flux conspaced electrodes '32, 34 and the insulative cover 36. In this form of augmentation sheath, a series of thin sheets of transformer iron 56 (of 16 mills or less thickness, for example) extend substantially perpendicularly of the electrodes 32, 34 and are interleaved with an even thinner insulating sheet 58. As will be apparent, the transformer iron sheets 56 each have the major dimension thereof extending across the direction of flow of the plasma and generally in planes of current fiow of the plasma are.

It is characteristic of all of the forms of flux concentrating sheaths shown in FIGS. 7, 7A and 7B that an increased magnetic flux is self-generated from the current flow in the electrodes 32, 34 and in the plasma are (current 28') in a direction to accelerate the plasma flow. It is a further important characteristic of the use of a ferromagnetic sheath to self-induce increased fiux density across the narrow dimension of the plasma zone that a high magnetic field is produced only at the point where the plasma is located at any given instant. There is therefore no high residual circuit conductance, i.e. the magnetic field produced is essentially entirely a useful field insofar as plasma acceleration is concerned. This improved mode of operation is to be distinguished from the proposition of applying an externally generated magnetic field, such as by externally excited coils or the like, or by external permanent magnets, since a continuously excited non-selfindnced field either wastes power (if electrically energized) or is unduly heavy (if a permanent magnet arrangement). Without a self-induced field, the field does not reverse when the plasma current reverses, and diminished or negative acceleration forces result.

An experimental accelerator constructed according to the present invention can for example employ an electrode spacing of 2 /2 inches with inch diameter elec trodes 1 foot in length, an insulative container providing a plasma channeling chamber inch thick, a capacitor of 15 microfarads, a 2.5 kv. voltage source, and a helium propellant. An accelerator so constructed, and operated in a pulsed on command mode, exhibited an efficiency (useful kinetic energy in the exhaust divided by the stored energy in the capacitor) of 48% at 5100 seconds specific impulse. With an appropriate ferromagnetic sheat 60 or 60 or 60" added, as shown in FIGS. 7, 7A and 7B, the efiiciency of the accelerator is increased to about 75% at the same specific impulse, and the magnetic field thereof is considerably more uniform, i.e. considerably more evenly distributed across the plasma channeling chamber. The flux density in the accelerating channel increases by a factor of about thirteen for any given circuit current as a result of the use of the electromagnetic sheath. The increase in efiiciency resulting from use of a flux concentrating ferromagnetic sheath becomes even more pronounced as the spacing between the electrodes 32, 34 is increased and as the interpole gap dimension is reduced. Optimizing circuit parameters and utilization of a ferromagnetic sheath in conjunction with a confined rail pulsed plasma accelerator according to the present invention, appears to have an efliciency capability as high as about side thereof.

95% of theoretical at about 5000 seconds specific impulse. Correspondingly, use of a ferromagnetic sheath makes possible efiiciencies of about 50% of theoretical at 1000 seconds specific impulse, whereas the efficiency capability otherwise is only about 15% of theoretical. Of notable importance, also is the fact that a ferromagneticsheath is practicably usable only in conjunction with a confined rail pulsed plasma accelerator, since parallel rails provide the only pulsed accelerator configuration compatible with use of opposed magnetic pole faces with a narrow gap therebetween.

Also of important practical significance is the fact that the length and therefore the weight of the ferromagnetic sheath and associated accelerating channel can be kept small because the sheath provides a high ratio of final to initial inductance. Engine miniaturization is quite prac tical with accelerator configurations according to the invention. For example, a typical confined rail micro-engine utilizing a ferromagnetic sheath can involve Vs inch diameter electrodes 4 inches long spaced 1 inch apart, a ferromagnetic sheath thickness of inch and a total channel weight of only /2 pound, for example.

FIGS. 8 and 9 illustrate a typical cluster of a plurality of pulsed plasma accelerators according to the present invention, arranged along diiferently directed thrust axes to provide selective thrust direction. Individual accelerators 66,: 68, 70 and 72 provide thrusting in four different directions along two perpendicular axes. Each accelerator includes the basic components of a confined parallel rail accelerator. Thus in accelerator 72 two elongate substantially parallel electrodes 74, 76 are surrounded by an insulative plasma channeling container 78. Like- Wise, in accelerator 70 there are two elongate substantially parallelelectrodes 80, 82 surrounded by an insulative plasma channeling container 84, in accelerator 68 there are rails 86, 88 and container 90, and in accelerator 66 there are rails 92, 94 and container 96. FIG. 8 further illustrates the typical interconnection of two or more such accelerator rail pairs to a single electrical energy storage capacitor 98. Electrodes 76, 82, 86, 92 are directly connected to one side of capacitor 98 and electrodes 74, 80, 88, 94 are directly connected to the other When the energy storage capacitor 98 is charged to a high voltage, each pair of electrodes in each accelerator 66, 68, 70 and 72 are charged with one elec trode at high voltage with respect to the other. Thrusting in the desired direction is obtained simply by admitting propellant into the proper accelerator. Such propellant admission might typically take the form of an on-off propellant admitting valve mounted on each accelerating channel, three of such valves being shown in FIGS. 8 and 9 at 100, 102, 104.

FIG. 9.is a side view of the accelerator cluster shown in FIG. 8, further showing the placement of propellant valves and the common electric energy storage capacitor 86 for all the accelerators 66, 68, 70 and 72. The four accelerators are connected to the single energy storagecapacitor 98 by a single conical coaxial connector 106. In addition to the four channels shown in FIG. 8 and likewise in FIG. 9, several more channels can be placed near the capacitor 98 and connected to it by appropriate parallel or other coaxial electrical leads.

FIG. 10 illustrates the typical use of such a plurality of pulsed plasma accelerator clusters for space craft attitude control. The individual accelerators 66, 68, 70, 72 and 108, 110 are clustered in such a manner as to provide directional attitude control for a typical space vehicle 112. Accelerators 66 and 70 provide pitch control, accelerators 68 and 72 provide yaw control, and accelerators 108 and 110 provide roll control for the space vehicle 112. All said accelerators can be connected to a single (i.e. common) energy storage capacitor, suitably mounted within the space vehicle body.

As earlier indicated, the confined parallel rail pulsed plasma accelerator can be operated in a continuous repetitive pulsing mode by the continuous admission of a stream of gaseous propellant and further that the accelerator operates on a wide variety of gaseous propellants, including air. These properties of the confined parallel rail pulsed plasma accelerator give rise to several advantageous operational possibilities: (1) operation within the earths atmosphere using the atmosphere directly as a propellant source, including the operation of the accelerator as a continuously selffed ramjet; (2) collection and condensation of the earths atmosphere while within the atmosphere, with use of the collected atmosphere as propellant for a part of or the remainder of the space mission; and (3) operation as in (1) and (2), further utilizing the atmosphere and/or surface materials of a foreign planet as the propellant source.

From the foregoing, various other modifications, accelerator arrangements, accelerator component arrangements, and accelerator operating techniques will occur to those skilled in the art to which the invention is addressed, within the scope of the following claims.

What is claimed is:

1. A pulsed plasma accelerator for generating propulsion thrust, comprising:

(a) a pair of spaced, elongate substantially parallel electrodes of substantially equal cross-sectional area;

(b) a high voltage power supply connected across said electrodes;

(c) an electrical energy storage capacitor connected across said electrodes;

(d) an insulative container surrounding said electrodes at the sides and at one end thereof, leaving the other end open, said electrodes being substantially coextensive with said container, with said container providing a plasma channeling chamber between said electrodes; and

(e) means for delivering and injecting a vaporizable and ionizable propellant into the closed end of said container.

2. The pulsed plasma accelerator of claim 1, wherein said means for delivering and injecting a vapcriza ble and ionizable propellant comprises means for varying the amount of propellant injected.

3. A pulsed plasma accelerator according to claim 1, wherein said means for delivering and injecting a vaporizable and ionizable propellant comprises means for varying the rate of propellant injection.

4. A pulsed plasma accelerator according to claim 3, wherein such propellant delivery and injection means comprises means injecting the propellant into the chamber continuously.

5. A pulsed plasma accelerator according to claim 4, wherein said propellant is a gas and said propellant delivery and injection means comprises a vibratory type flow modulator, the continuous injection of the gas into the container being characterized by a cyclic pulsation.

6. A pulsed plasma accelerator according to claim 1, wherein said vaporizable and ionizable propellant is a gas.

7. A pulsed plasma accelerator according to claim 6, wherein said gas is selected from the group consisting of ammonia and water vapor.

8. A pulsed plasma accelerator comprising spaced electroconductive rails of substantially equal cross-sectional area within an insulative container providing a plasma channeling chamber closed at one end and open at the other with said rails being substantially coextensive with said chamber, the said chamber being laterally of a width considerably greater than the lateral thickness thereof, with the sides of the container being substantially fiat and closely spaced with respect to the rails, said accelerator further comprising means including an energy storage capacitor for charging and recharging one of said rails to a high voltage with respect to the other, and means for injecting a vaporizable and ionizable propellant into the closed end of said container.

9. The method of operating a pulsed plasma accelerator of the type having means for injecting a vaporizable and ionizable propellant between spaced electroconductive rails enclosed in a relatively thin plasma channeling chamber open at one end and closed at the other, said method comprising: continuously injecting the propellant into the closed end of said chamber; and continuously applying to an energy storage capacitor connected across said rails a relative potential difference of sufficient magnitude to cause self-pulsed ionization and acceleration of the propellant.

10. The method of pulsed plasma accelerator operation set forth in claim 9, comprising cyclically varying the rate of propellant injection into the chamber.

11. The method of pulsed plasma accelerator operation set forth in claim 10, wherein the cyclic variation in rate of propellant injection occurs at a frequency on the order of about one thousand cycles per second.

12. The method of pulsed plasma accelerator operation set forth in claim 9, wherein said vaporizable and ionizable propellant is a gas.

13. A method of pulsed plasma accelerator operation set forth in claim 12, wherein said gas is selected from the group consisting of ammonia and water vapor.

14. A space craft attitude control system comprising a plurality of plasma accelerators arranged along differently directed thrust axes to provide selective control of the space craft attitude, each such accelerator comprising a pair of spaced electroconductive rails within an insulative container closed at one end and open at the other, the enclosed chamber between the rails being laterally of a width considerably greater than the lateral thickness thereof, a single energy storage capacitor connected in common with one rail of each of the plurality of rail pairs to charge and recharge one of the rails of each such plurality of rail pairs to a high potential with respect to the other rail of the pair, and means for delivering and separately injecting a vaporizable and ionizable propellant into the closed end of each of the insulative containers.

15. A space craft attitude control system according to claim 14, wherein a plurality of rails, each being one rail of a different rail pair, constitute a single structural, electroconductive unit.

16. A space craft attitude control system according to claim 14, wherein each of the means for delivering and injecting a vaporizable and ionizable propellant into the respective insulative containers comprises a separately controllable injection means associated with each container, the selective opening of each such injection means causing thrust to be generated by the associated acceleratOl'.

17. A pulsed plasma accelerator for generating propulsion thrust, comprising:

(a) a pair of spaced, elongate substantially parallel electrodes;

(b) a high voltage power supply connected across said electrodes;

() an electrical energy storage capacitor connected across said electrodes;

(d) an insulative container surrounding said electrodes at the sides and at one end thereof, leaving the other end open, said container providing a plasma channeling chamber between said electrodes extending substantially the entire length thereof;

(e) a ferromagnetic flux concentrator laterally surrounding said insulative container in a manner placing magnetizable pole pieces substantially against the side faces of said container so that plasma current self-induces an increased flux density within the ionized plasma, with the flux lines thereof paralleling the thickness dimension of said container; and

(f) means for delivering and injecting a vaporizable and ionizable propellant into the closed end of said container.

18. A pulsed plasma accelerator as set forth in claim 17, wherein said ferromagnetic flux concentrator is composed essentially of a laminate of ferrous sheet material with interleaved insulative sheet material, the major dimension of the ferrous sheet material extending generally laterally of the direction of plasma movement in said plasma channeling chamber.

19. A pulsed plasma accelerator as set forth in claim 17, wherein said ferromagnetic flux concentrator is fabricated essentially of a resin impregnated with particulate magnetic material.

20. A pulsed plasma accelerator as set forth in claim 17, wherein said ferromagnetic flux concentrator comprises compacted iron particles with an external protective coating.

21. In a pulsed plasma accelerator wherein a vaporizable and ionizable propellant is injected into a plasma accelerating chamber open only at one end; the improvement in chamber configuration wherein said chamber in a cross-section thereof lateral to the direction of plasma movement is generally rectangular in configuration and bounded by electroconductive surfaces and insulative surfaces in alternating pattern, the total surface area of said insulative surfaces being substantially greater than the total surface area of said electroconductive surfaces.

References Cited by the Examiner UNITED STATES PATENTS 2,961,559 11/1960 Marshall 31363 2,974,594 '3/1961 Boehm 35.54 3,024,596 3/196'2 Hatfield 6035.54 3,149,459 9/1964 Ulam 2 6035.5

OTHER REFERENCES Propulsion Systems for Space Flight (Corliss) Published by McGraw-Hill Book Company (New York) 1960 (pages 158, 159 and 215-220 relied on). (Copy in Scientific Library and in Group 370.)

Plasma Acceleration (Ka-sh) Published by Stanford University Press (Stanford, California) 1960, pages 60-70 relied on. (Copy in Scientific Library and Group 370.)

Advanced Propulsion Techniques (Penner) Published by Pergamon Press (New York) 1961 (pages 123 127 )relied on). (Copy in Scientific Library and Group 370.

Proceedings of the I.R.E., vol. 49, No. 12. December 1961 (pages 1817 and 1818 relied on). (Copy in Scientific Library and Group 370.)

CARLTON R. CROYLE, Primary Examiner. MARK M. NEWMAN, Assistant Examiner. 

1. A PULSED PLASMA ACCELERATOR FOR GENERATING PROPULSION THRUST, COMPRISING: (A) A PAIR OF SPACED, ELONGATE SUBSTANTIALLY PARALLEL ELECTRODES OF SUBSTANTIALLY EQUAL CROSS-SECTIONAL AREA; (B) A HIGH VOLTAGE POWER SUPPLY CONNECTED ACROSS SAID ELECTRODES; (C) AN ELECTRICAL ENERGY STORAGE CAPACITOR CONNECTED ACROSS SAID ELECTRODES; (D) AN INSULATIVE CONTAINER SURROUNDING SAID ELECTRODES AT THE SIDES AND AT ONE END THEREOF, LEAVING THE OTHER END OPEN, SAID ELECTRODES BEING SUBSTANTIALLY COEXTENSIVE WITH SAID CONTAINER, WITH SAID CONTAINER PROVIDING A PLASMA CHANNELING CHAMBER BETWEEN SAID ELECTRODES; AND (E) MEANS FOR DELIVERING AND INJECTING A VAPORIZABLE AND IONIZABLE PROPELLANT INTO THE CLOSED END OF SAID CONTAINER. 